MacBook – Reinstall Office for Mac 2011

Office 2011 requires activation and is tied to your hardware so it might very well be that you need to reactivate Office 2011 after the HD change. The whole activation procedure is described in Office for Mac 2011 Activation and Product key Information (via archive.org). According to this document, the product key was mailed to you in a mail with subject "Microsoft Office for Mac – Order Confirmation".

Microsoft has replaced the support article linked above with a new one describing how to Activate Office for Mac 2011. As mentioned on the top of that page

Support for Office for Mac 2011 ended on October 10, 2017. All of your Office for Mac 2011 apps will continue to function. However, you could expose yourself to serious and potentially harmful security risks. Upgrade to a newer version of Office so you can stay up to date with all the latest features, patches, and security updates.

Since you say it's a legitimate copy (I'm assuming retail—if this is volume licensing, that's a whole other kettle of fish), one path that's open to you is contacting Microsoft.

They are equipped to deal with lost product keys, and in most cases can help you retrieve it. Worst case, they can sell you a new one at a steep discount.

See: How to locate your product key or obtain a new product key for Office for Mac.

(And for the future, you might want to stash your product keys and serial numbers away for safekeeping. 1Password has a category just for this. If you don't use 1Password, Keychain Access's "Secure Note" functionality can be put to good use.)